Antenna selection with frequency-hopped sounding reference signals

ABSTRACT

A transceiver has a first antenna and a second antenna for transmitting alternatively a frequency-hopped sounding reference signal (SRS) over a sub-band of a bandwidth at a time. The transceiver includes a determination unit for determining whether a number of sub-bands in the bandwidth is odd or even, a transmitter for transmitting the SRS continuously from the first antenna, if the number of sub-bands is even, and a receiver for receiving a response to the transmitting.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/415,578 submitted by Mehta et al. on Mar. 31, 2009 for “AntennaSelection with Frequency-Hopped Sounding Reference Signals.”

FIELD OF THE INVENTION

This invention relates generally to antenna selection in wirelesscommunication, networks, and more particularly to selecting antennaswith frequency-hopped sounding reference signals.

BACKGROUND OF THE INVENTION

OFDMA and SC-OFDMA

In a wireless communication network, such as the 3^(rd) generation (3G)wireless cellular communication standard and the 3GPP long termevolution (LTE) standard, it is desired to concurrently support multipleservices and multiple data rates for multiple users in a fixed bandwidthchannel. One scheme adaptively modulates and codes symbols beforetransmission based on current channel estimates. Another optionavailable in LTE, which uses orthogonal frequency division multiplexedaccess (OFDMA), is to exploit multi-user frequency diversity byassigning different sub-carriers or groups of sub-carriers to differentusers or UEs (user equipment, mobile station or transceiver), in thesingle carrier frequency division multiple access (SC-FDMA) uplink ofLTE, in each user, the symbols are first together spread by means of aDiscrete Fourier Transform (DFT) matrix and are then assigned todifferent sub-carriers. The network bandwidth can vary, for example,from 1.25 MHz to 20 MHz. The network bandwidth is partitioned into anumber of subcarriers, e.g., 1.024 subcarriers for a 10 MHz bandwidth.

The following standardization documents are applicable 36.211, 3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network; Physical Channels and Modulation (Release 8), v 1.0.0(2007-03); R1-01057, “Adaptive antenna switching for radio resourceallocation in the EUTRA uplink,” Mitsubishi Electric/Nortel/NTT DoCoMo,3GPP RAN1#48, St. Louis, USA; R1-071119, “A new DM-RS transmissionscheme for antenna selection in E-UTRA uplink,” LGE, 3GPP RAN1#48, St.Louis, USA; and “Comparison of closed-loop antenna selection withopen-loop transmit diversity (antenna switching within a transmit timeinterval (TTI),” Mitsubishi Electric, 3GPP RAN1#47bis, Sorrento, Italy.According to the 3GPP standard, the base station (BS) is enhanced, andis called the “Evolved NodeB” (eNodeB). We use the terms BS and eNodeBinterchangeably.

Multiple Input Multiple Output (MIMO)

In order to further increase the capacity of a wireless communicationnetwork in fading channel environments, multiple-input-multiple-output(MIMO) antenna technology can be used to increase the capacity of thenetwork without an increase in bandwidth. Because the channels fordifferent antennas can be quite different, MIMO increases robustness tolading and also enables multiple data streams to be transmittedconcurrently.

Moreover, processing the signals received in spatial multiplexingschemes or with space-time trellis codes requires transceivers where thecomplexity can increase exponentially as a function of the number ofantenna.

Antenna Selection

Antennas are relatively simple and cheap, while RF chains areconsiderably more complex and expensive. Antenna selection reduces someof the complexity drawbacks associated with MIMO networks. Antennaselection reduces the hardware complexity of transmitters and receiversin the transceivers by using fewer RF chains than the number ofantennas.

In antenna selection, a subset of the set of available antennas isadaptively selected by a switch, and only signals for the selectedsubset of antennas are connected to the available RF chains for signalprocessing, which can be either transmitting or receiving. As usedherein, the selected subset, in all cases, means one or more of all theavailable antennas in the set of antennas.

Antenna Selection Signals

Pilot Tones or Reference Signals

In order to select the optimal subset of antennas, channelscorresponding to available subsets of antennas need to be estimated,even though only a selected optimal subset of the antennas is eventuallyused for transmission.

This can be achieved by transmitting antenna selection signals, e.g.,pilot tones, also called reference signals, from different antennas orantenna subsets. The different antenna subsets can transmit either thesame pilot tones or use different ones. Let N_(t) denote the number oftransmit antennas, N_(r) the number of receive antennas, and letR_(t)=N_(t)/L_(t) and R_(r)=N_(r)/L_(r) be integers. Then, the availabletransmit (receive) antenna elements can be partitioned into R_(t)(R_(r)) disjoint subsets. The pilot repetition approach repeats, forR_(t)×R_(r) times, a training sequence that is suitable for anL_(t)×L_(r) MIMO network. During each repetition of the trainingsequence, the transmit RF chains are connected to different subsets ofantennas. Thus, at the end of the R_(t)×R_(r) repetitions, the receiverhas a complete estimate of all the channels from the various transmitantennas to the various receive antennas.

In case of transmit antenna selection in frequency division duplex (FDD)networks, in which the forward and reverse links (channels) are notidentical, the transceiver feeds back the optima set of the selectedsubset of antennas to the transmitter. In reciprocal time divisionduplex (TDD) networks, the transmitter can perform the selectionindependently.

For indoor local area network (LAN) applications with slowly varyingchannels, antenna selection can be performed using a media access (MAC)layer protocol, see IEEE 802.11n wireless LAM draft specification, 1.P802.11n/D1.0, “Draft amendment to Wireless LAN media access control(MAC) and physical layer (PHY) specifications: Enhancements for higherthroughput,” Tech. Rep., March 2006.

Instead of extending the physical (PHY) layer preamble to include theextra training fields (repetitions) for the additional antennas, antennaselection training is done at the MAC layer by issuing commands to thephysical layer to transmit and receive packets by different antennasubsets. The training information, which is a single standard trainingsequence for an L_(t)×L_(r) MIMO network, is embedded in the MAC headerfield.

SC-FDMA Structure in LTE

The basic uplink transmission scheme is described in 3GPP TR 25.814,v1.2.2 “Physical Layer Aspects for Evolved UTRA.” The scheme is asingle-carrier transmission (SC-FDMA) with cyclic prefix (CP) to achieveuplink inter-user orthogonality and to enable efficient frequency-domainequalization at the receiver.

Broadband Sounding Reference Signals (SRS)

The broadband SRS helps the eNodeB to estimate the entire frequencydomain response of the uplink channel from the user to the eNodeB. Thishelps frequency-domain scheduling, in which a subcarrier is assigned, inprinciple, to the user with the best uplink channel gain for that,subcarrier. Therefore, the broadband SRS can occupy the entire networkbandwidth, e.g., 5 MHz or 10 MHz, or a portion thereof as determined bythe eNodeB. In the latter case, the broadband SRS is frequency hoppedover multiple transmissions in order to cover the entire networkbandwidth.

SUMMARY OF THE INVENTION

The embodiments of the invention describe a method for antenna selectionin a wireless communication network. The network includes a transceiverhaving a set of antennas. The transceiver is configured to transmit afrequency-hopped sounding reference signal (SRS) over a subband from asubset of antennas at a time. The transceiver transmits thefrequency-hopped SRS from subsets of antennas in the set of antennasalternately. In response to the transmitting, the transceiver receivesinformation indicative of an optimal subset of antennas and transmitsdata from the optimal subset of antennas.

In some embodiments, we assign an index for each subset of antennas. Wealso use the ‘selected’ and ‘unselected’ subset of antennas as anindication to select particular subset of the antennas by thetransceiver for the transmission. The index of the selected subset ofantennas a(n_(SRS)) depends on the subframe number N_(SRS) in which theSRS is transmitted and a number of the subset of antennas. Therefore,the index pattern above can be specified in the form a functionalrelationship between a(N_(SRS)) and n_(SRS).

In one embodiment, the transceiver has two subsets of antennas, and theindexes are 0 and 1. Accordingly, the transmitting alternately leads toan index pattern of the selected subset of antennas [0, 1, 0, 1, 0, 1,0, 1 . . . ]. In another embodiment, the transceiver has three subsetsof antennas, the index pattern of the selected subset of antennas [0, 1,2, 0, 1, 2, 0, 1, 2, 0, 1, 2 . . . ]. In various embodiments, we areswitching the index of the selected subset of antennas every time afterthe transmitting the frequency-hopped SRS.

Accordingly, one embodiment of the invention discloses a transceiverhaving a first antenna and a second antenna for transmittingalternatively a frequency-hopped sounding reference signal (SRS) over asub-band of a bandwidth at a time. The transceiver includes adetermination unit for determining whether a number of sub-bands in thebandwidth is odd or even; a transmitter for transmitting the SRScontinuously from the first antenna, if the number of sub-bands is even;and a receiver for receiving a response to the transmitting.

Another embodiment discloses a method for antenna selection (AS) in atransceiver having a number of subsets of antennas for transmitting afrequency-hopped sounding reference signal (SRS) over a sub-band of abandwidth from a subset of antennas at a time. The method includes adetermining whether a number of sub-bands in the bandwidth is an integermultiplier of the number of subsets of antennas; and transmitting theSRS alternately from the subsets of the antennas if the number ofsub-bands is not the integer multiplier of the subsets of antennas, andfor transmitting the SRS substantially alternatively, if the number ofsub-bands is tire integer multiplier of the subsets of antennas, whereinthe transmitting substantially alternatively includes transmitting theSRS continuously from the same subset of antennas.

Yet another embodiment discloses a transceiver having a first antennaand a second antenna for transmitting alternatively a frequency-hoppedsounding reference signal (SRS) over a sub-band of a bandwidth at atime. The transceiver includes a processor for determining whether anumber of sub-bands in the bandwidth is odd or even, for switching anindex of a subset of antennas transmitting die SRS every time after thetransmitting, and, if the number of sub-bands is even, for periodicallyaltering the index to include a continuous SRS transmission from thesame subset of antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wireless network according to anembodiment of the invention;

FIG. 2 is a block diagram of an uplink resource grid according to anembodiment of the invention;

FIG. 3 is a block diagram of a resource block according to an embodimentof the invention;

FIG. 4 is a block diagram of method for selecting antennas according toan embodiment of the invention;

FIGS. 5 and 6 are block diagrams of a frequency-hopped soundingreference signal (SRS) transmission;

FIG. 7 is a block diagram of a method and a network for training subsetsof antennas with, the frequency-hopped SRS according to embodiments ofthe invention; and

FIGS. 8 and 9 are block diagrams of a frequency-hopped sound referencesignal (SRS) transmission according to embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

LTE Network Overview

FIG. 1 shows the general structure of an LTE wireless network accordingto an embodiment of the invention. Multiple user equipments (UEs) ormobile transceivers 111-113 communicate with a stationary base station(BS) 110. The base station also includes transceivers.

The base station is called an evolved Node B (eNodeB) in the LTEstandard. The eNodeB 110 manages and coordinates all communications withthe transceivers in a cell using wireless channels or connections 101,102, 103. Each connection can operate as a downlink (DL) from the basestation to the transceiver or an uplink from the transceiver to the basestation. Because the transmission power available at the base station isorders of magnitude greater than the transmission power at thetransmitter, the performance on the uplink is much more critical.

To perform wireless communication, both the eNodeB and the transmittersare equipped with at least one RF chain and a number of antennas.Normally, the number of antennas and the number RF chains are equal atthe eNodeB. The number of antennas at the base station can be quitelarge, e.g., eight. However, due to the limitation on cost, size, andpower consumption, mobile transceivers usually have less RF chains thanantennas 115. The number of antennas available at the transceiver isrelatively small, e.g., two or four, when compared with the basestation. Therefore, antenna training and selection as described isapplied at the transceivers.

During operation, the transceiver switches the antennas between transmitRF chain(s) to transmit. Generally, antennas selection selects a subsetof antennas from a set of available antennas at the transceiver. Theantennas selection includes the training, which is used for generatingand transmitting and receiving antenna selection signals. Theembodiments of the invention enable the network to accommodatetransceivers with different SRS bandwidths in an orthogonal manner, anduse the limited resource of SRS sequences well.

LTE Frame Structure

The uplink (transceiver to eNodeB) and downlink (eNodeB to transceiver)transmissions are organized into radio frames. A radio frame is 10 mslong, and consists of 20 slots 306 of duration 0.5 ms each. Twoconsecutive slots constitute a subframe 301. The frame includes twentysubframes in the time domain.

FIG. 2 shows the baste structure of a SC-FDMA (single carrier frequencydivision multiple access) uplink resource grid 200. The horizontal axisindicates time or SC-FDMA symbols and the vertical axis indicatesfrequency or subcarriers. The number of subcarriers depends on thenetwork bandwidth, which can range from 1.25 MHz to 20 MHz for example.

The uplink resource grid consists of resource elements. Each resourceelement is indemnified by the subcarrier and the SC-FDMA symbol. Theresource elements are grouped into resource blocks. A resource block(RB) consists of 12 consecutive subcarriers and six or seven consecutiveSC-FDMA symbols in time. The number of SC-FDMA symbols depends on thecyclic prefix (CP) length. For a normal cyclic prefix, the number ofSC-FDMA symbols equals 7 and for an extended cyclic prefix, the numberof SC-FDMA symbols equals 6.)

Each subframe constitutes a resource block, see inset 300 and FIG. 3 fordetails. For the purpose of this specification and appended claims, weuse the terms the subframe and the transmission time interval (TTI)interchangeably.

FIG. 3 shows a structure of a resource block (RB) 300 for the normalcyclic prefix. The vertical axis indicates frequency, and the horizontalaxis time. In frequency domain, the resource block includes of a numberof subcarriers. In time domain, the RB is partitioned into SC-FDMAsymbols, which may include data 203 and reference signals (RS) 210. Twotypes of the RS are used in the uplink: sounding reference signals (SRS)311 and demodulation reference signals (DMRS) 310.

Both the SRS and the DMRS are generated using a constant amplitude zeroautocorrelation sequence (CAZAC) sequence such as a Zadoff-Chu sequence,as explained in Section 5.5.1 of TS 36.211 v8.5.0 incorporated herein byreference. When the sequence length is not equal to the length possiblefor a Zadoff-Chu sequence, the sequence of desired length is generatedby extending circularly a Zadoff-Chu sequence of length close to andless than the desired length or by truncating a Zadoff-Chu sequence oflength close to and greater than the desired length. The DMRS istransmitted in the fourth SC-FDMA symbol for normal cyclic prefix and inthe third SC-FDMA symbol for the extended cyclic prefix. The SRS, whentransmitted, is typically transmitted in the last SC-FDMA symbol of thesubframe except for special subframes as described in TS 36.211 v.8.5.0.However, the embodiments of the invention do not depend on the SC-FDMAsymbol in which the RS is transmitted.

Antennas Selection

Typically, the RS is transmitted along with or separately from user datafrom different subsets of antennas. Based on the RSs, the base station,estimates channels and identifies the optimal subset, of antennas fordata transmission.

FIG. 4 shows the basic method for antenna selection according to anembodiment of the invention. The base station 110 specifies instructions151, e.g., frequency-hopped pattern and subsets of antennas to use fortransmitting RSs 161. The transceiver 101 transmits the RSs 161according to the instructions 151.

The base station selects 170 a subset of antennas 181 based on thereceived RSs. Then, the base station indicates 180 the selected subsetof antenna 181 to the transceiver. Subsequently, the transceiver 101transmits 190 data 191 using the selected subset of antennas 181. Thetransceiver can also use the same subset of antennas for receivingtransmitting data.

Sounding Reference Signal (SRS)

The SRS is usually a wideband or variable bandwidth signal. The SRSenables the base station to estimate the frequency response of theentire bandwidth available for the network, or only a portion thereof.This information enables the base station to perform resource allocationsuch as uplink frequency-domain scheduling. According to the embodimentof the invention, the SRSs are also used for antenna selection.

Another option for LTE is to use the frequency-hopping (FH) pattern totransmit the SRS. Specifically, a hopping SRS, with a bandwidth smallerthan the network bandwidth, i.e., a subband, is transmitted based on apre-determined frequency hopping pattern. The hopped SRSs, over multipletransmissions, span a large portion of the entire bandwidth availablefor the network, or even the entire available bandwidth. With frequencyhopping, the probability that transceivers interfere with each otherduring training is decreased.

However, if performed incorrectly, antenna selection with afrequency-hopped variable bandwidth SRS results in limited, performanceimprovement, particularly if the transceiver is moving rapidly. Forexample, as shown on FIG. 5, all the subbands of antenna Tx₁ aresuccessively sounded by a frequency-hopped SRS. Thereafter, the subbandsof antenna Tx₂ are successively sounded in a similar manner, as shown bythe shaded blocks. However, the channel estimates obtained from thisfrequency-domain antenna selection training pattern rapidly becomesoutdated.

FIG. 6 shows subframes with frequency-hopped SRS transmitted fromavailable subsets of antennas alternately. For example, the transceivertransmits the SRS alternately from two subsets of antennas, i.e., Tx1210 and Tx2 220. The available bandwidth 240 is partitioned into foursubbands 241-244, such that the SRS covers the bandwidth with fourtransmissions 250. Please note, that a subband can occupy one ormultiple RB.

As can be seen from FIG. 6, in this transmission scenario, the SRSs forthe subbands 241 and 243 are always transmitted from the subset ofantennas Tx1, and the SRSs for the subbands 242 and 244 are alwaystransmitted from the subset of antennas Tx2. Hence, the transceiver isnot able to estimate the channel over entire frequency domain for eachavailable subset of antennas.

FIG. 7 shows a method and a network 700 for training for the subset ofantennas with the frequency-hopped SRS transmitted from the subsets ofantennas according to embodiments of the invention. Transmittingsubstantially alternately means, as define herein for the purpose ofthis specification and appended claims, that the SRSs are transmittedfrom each subset in the set of antennas alternately, but periodically anorder of the subsets schedule for the transmission is altered.

In some embodiments, we assign an index for each subset of antennas. Wealso use the ‘selected’ and ‘unselected’ subset of antennas as anindication to select particular subset of the antennas by thetransceiver for the transmission.

For example, if the transceiver has two subsets of antennas, the indexeswill be 0 and 1. Accordingly, the transmitting alternately leads to anindex pattern of the selected subset of antennas [0, 1, 0, 1, 0, 1, 0, 1. . . ]. If the transceiver use more than two subsets of antennas forthe transmission, all the subsets of antennas transmit the SRS signalsalternately according indexes of the subset. For example, if thetransceiver has three subsets of antennas, the index pattern of theselected subset of antennas [0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2 . . . ].Thus, we are switching the index of the selected subset of antennasevery time after the transmitting the frequency-hopped SRS.

However, the transmitting substantially alternately leads to an indexpattern, e.g., [0, 1, 0, 1, 1, 0, 1, 0, 0, 1 . . . ]. Please note, thatfor the transmitting substantially alternately method we periodicallyalter the index for die transmitting subset, e.g., shift or omit theindexes. The index of the selected subset of antennas a(n_(SRS)) dependson the subframe number n_(SRS) in which the SRS is transmitted and anumber of the subset of antennas. Therefore, the index pattern above canbe specified, in the form a functional relationship between a(n_(SRS))and n_(SRS), The functional relationship depends on other parameterssuch as, but not limited to, the base station index and the length ofthe SRS sequence.

We determine 740 a type of a transmission based on a relationshipbetween the number of subbands 710 in the bandwidth and the number ofthe subsets of transmit antennas 720 to be trained. As described indetails below, if 730 the number of subbands is an integer multiplier ofthe number of transmit antennas 731, we transmit the SRSs substantiallyalternately 760. For example, we switch an antenna index 750 every timewhen the end of bandwidth is reached 735. In alternative embodiment, weswitch antenna index after the end or at the beginning of thefrequency-hopped pattern, if 730 the number of subbands is not integermultiplier of the number of transmit antennas 733, we transmit the SRSsalternately.

FIG. 8 shows a diagram for a method for transmitting alternately thefrequency-hopped SRS. The available bandwidth of B Hz 810 is split intoN_(f) 830 subbands of bandwidth

$\frac{B}{N_{f}}$Hz each. If number of subbands is odd, e.g., N_(f)=5, and the number ofthe subsets of antennas is even, e.g., two, then the number of subbandsis not integer multiplier of the number of transmit antennas. Thus, thetransmission from the two antennas, Tx1 and Tx2, alternately results ina time-interleaved frequency hopping pattern.

FIG. 9 shows a diagram for a method for transmitting substantiallyalternately the frequency-hopped SRS. In this embodiment, the number ofsubbands, i.e., tour, is integer multiplier of the number oftransmitting antennas, i.e., two. Accordingly, when the transmissionreaches the end of the bandwidth, e.g., a pattern of transmissions 920,we switch the indexes of the subset of the antennas. Thus, the nextpattern of transmissions 930 starts from the subset of antennas Tx2,instead of the subset Tx1 as for the cycle 920.

As described above, in one embodiment, the decision of which trainingpattern to use is made by the base station. The training pattern istransmitted to the transceiver as part of the instruction 151. Inalternative embodiment, the transceiver has knowledge about the possibletraining patterns, and the instruction 151 includes only identificationof the training pattern to use.

Although the invention has been described by way of examples ofpreferred embodiments, it is to be understood that various otheradaptations and modifications may be made within the spirit and scope ofthe invention. Therefore, it is the object of the appended claims tocover all such variations and modifications as come within the truespirit and scope of the invention.

We claim:
 1. A method for antenna selection (AS) in a transceiver havinga number of subsets of antennas for transmitting a frequency-hoppedsounding reference signal (SRS) over a sub-band of a bandwidth from asubset of antennas at a time, comprising: determining whether a numberof sub-bands in the bandwidth is an integer multiplier of the number ofsubsets of antennas; and transmitting the SRS alternately from thesubsets of the antennas if the number of sub-bands is not the integermultiplier of the subsets of antennas, and transmitting the SRSsubstantially alternatively, if the number of sub-bands is the integermultiplier of the subsets of antennas, wherein the transmittingsubstantially alternatively includes transmitting the SRS alternativelyand periodically switching an order of transmission to continuouslytransmit two SRSs from the same subset of antennas.